Loss of upper airway muscle activity during sleep is thought to have an important role in the pathogenesis of obstructive sleep apnea. However, because of the differences in upper airway muscle activity between wakefulness and sleep, studies of the upper airway during wakefulness are not capable of predicting the site of upper airway closure during sleep. Airway closure during sleep in patients with sleep apnea can occur at different sites in the pharyngeal airway (36, 60, 61). The primary site of closure is most frequently located in the velophar-ynx but can also occur in the oro- and/or hypopharynx. Determination of the site of airway closure is important, as studies indicate that it determines the outcome of uvulopalatopharyn-goplasty (62, 63). Tests that determine the site of upper airway closure during sleep cannot be used to assess upper airway muscle function, as airway closure during sleep is also dependent on other factors, including the transmission of subatmo-spheric intrathoracic pressure into the pharyngeal airway and closing pressure (i.e., the pressure at which the pharyngeal airway closes in the absence of upper airway muscle activity and respiratory effort) (15, 27). Factors that narrow the pharyngeal airway raise closing pressure. Normally, closing pressure is subatmospheric. In patients with OSA, closing pressure is usually positive, that is, a positive intralumenal pressure is required during sleep to maintain pharyngeal airway patency. The site of airway closure can be determined during sleep with the placement of catheters attached to pressure transducers in the esophagus and at different levels in the pharyngeal airway (57). The site of pharyngeal airway closure during sleep can also be determined by imaging the upper airway with a fiberoptic scope or by fast acquisition MRI or computed axial tomography scans (36, 64).
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